1. Field of the Invention
This invention generally relates to a projection type display apparatus, which is also called a projection display, and an image display system, and more particularly to a projection type display apparatus and an image display system for enlarging and projecting an image formed by rays of light guided to a projection optical unit by, for example, a digital micro device (hereinafter referred to as DMD).
2. Description of the Related Art
In recent years, a display apparatus with a large screen has also come into use in general households, and a market demand has been made for a display apparatus having a screen size of 40 inches or more, which brings about manufacturing difficulties in the form of a CRT. Therefore, in order to meet this demand, a projection display, or the like, has been used as a display having a screen size of 40 inches or more.
The projection display has a conventional structure in which divided rays of light of R(red), G(green), B(blue), etc., arc irradiated to a small optical device, such as a liquid crystal panel or a DMD, from a high-pressure mercury lamp, a halogen lamp, or a similar lamp (hereinafter, these lamps are each referred to as a light source), and an image output from this optical device is enlarged and irradiated onto a screen by use of a projection optical unit formed as an image formation system that is comprised of a projection lens and a reflection optical element.
In the projection optical unit of the conventional projection display, abovementioned optical elements that form a group and that constitute the projection optical unit are contained in one case in order to prevent the occurrence of errors in the relative position and in the angle between the optical elements. If a DMD is used as the optical device, a brightness-reversed image is output in a direction opposite to a display image that is output in response to light that is input to the DMD from the light source, for structural reasons of the DMD.
Herein, the light used to form the reversed image is unnecessary light that is not guided to the projection optical unit. However, when this unnecessary light is irradiated onto the case and is transformed into thermal energy, the light raises the temperature of the case containing the projection optical unit, and causes the following problems.
That is, with the rise in temperature, the case expands, and, accordingly, errors become great in the relative position and in the angle between the optical elements that constitute the projection optical unit. The positional and angular errors of the optical elements greatly affect an image to be displayed. For example, the errors distort and roughen the quality of an image that is enlarged and projected onto the screen. In addition, if the unnecessary light that has not been intercepted in the case is diffused therein, a ghost occurs, thus deteriorating the image quality. For example, the ghost affects the contrast of an image to be displayed.